Progressive lens
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Progressive lenses, also called progressive addition lenses (PAL), progressive power lenses, graduated lenses, no-line bifocals, and varifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterised by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors. The gradient starts at a minimum, or no addition power, at the top of the lens and reaches a maximum addition power, magnification, at the bottom of the lens. The length of the progressive power gradient on the lens surface is usually between 15 and 20 mm with a final addition power between 1.00 to 2.50 dioptres for most wearers. The addition value prescribed depends on the level of presbyopia of the patient and is closely related to age.
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[edit] History
The first patent for a PAL was British Patent 15,735, granted to Own Aves with a 1907 priority date. Aves' patent included the progressive lens design and the manufacturing process. However this was unlike modern PALs. It consisted of a conical back surface and a cylindrical front with opposing axes in order to create a power progression. This design was never commercialized.
While there were several intermediate steps (H. Newbold appears to have designed a similar lens to Aves around 1913), there is evidence[citation needed] to suggest that Duke Elder in 1922 developed the worlds first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal". This was based on an arrangement of aspherical surfaces.
The Varilux lens was the first PAL of modern design. It was developed by Bernard Maitenaz, patented in 1953, and introduced by the Société des Lunetiers (now called Essilor) in 1959. Varilux is currently the most widely sold progressive lens brand worldwide.
Early progressive lenses were relatively crude designs but modern sophisticated progressive lenses have gained greater patient acceptance and include special designs to cater to many separate types of wearer application: for example lenses may be customized for use with computers, or to offer enlarged near and intermediate view areas. Over the 1980s through today, manufacturers have been able to minimize unwanted aberrations by:
- Improvements in mathematical modeling of surfaces, allowing greater design control.
- Extensive wearer trials.
- Improved manufacturing and lens metrology technology.
Today the complex surfaces of a progressive lens can be cut and polished on computer-controlled machines, allowing 'freeform surfacing', as opposed to the earlier casting process.
[edit] Advantages and use
- The wearer can adjust the additional lens power required for clear vision at different viewing distances by tilting his or her head to sight through the appropriate part of the vertical progression;
- The lens location of the correct addition power for the viewing distance usually only requires small adjustments to head position, since near vision tasks such as reading are usually low in the visual field and distant objects higher in the visual field.
- Progressive addition lenses avoid the discontinuities (image-jumps) in the visual field created by bifocal and trifocal lenses and are more cosmetically attractive. Since bifocal and related designs are associated with 'old age', proponents have suggested the lack of segments on the lens surface of a progressive lens appears more 'youthful' since lenses associated with younger wearers [single vision] lenses tend to be free of segments or lines on the surface.
[edit] Disadvantages
Distortion: Progressive lenses suffer the disadvantage that the power progression creates regions of aberration away from the optic axis, yielding poor visual resolution (blur). As the lenses combine a range of powers in a single surface there are also geometric distortions to the visual field, which increase with the addition power. Some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs. However, manufacturers claim acceptance rates of 90%-98%. Clinicians generally agree that in order to avoid adaptation problems it is best to start wearing progressive lenses early in the development of presbyopia (around 45 years of age for most people) while the prescribed addition powers are low. The wearer can then adapt to the increases in a series of steps in addition power over a number of years as their presbyopia progresses.
Fitting: Progressive lenses require careful placement relative to the wearers pupil centre for a distance'-viewing reference position. Incorrect specification of the fitting location can cause problems for the wearer including narrow fields of view, clear vision in one eye only, on-axis blur, and the need to adopt uncomfortable head positions.
Cost: Progressive lenses are sold at a significantly higher price than bifocal and single-vision reading spectacles.
Industry practitioners will never use the term "distortion" when describing the useless area of a progressive lens. Instead they will opt for the following terms: surface astigmatism, aberration, blended areas, sand, soft focus, image swim, blurring, less clear, and my personal favorite, peripheral transition areas. Make no mistake however, if you have a strong prescription, you will be moving your head back and forth like you're watching a tennis match, to read a 8.5x11 piece of paper.
[edit] Other notes
- For those new to progressive lenses, an accommodation period is required as the brain needs to learn to work with them. This period varies from a few days for some individuals to a couple of weeks. During this period, side effects can include headache and dizziness. It is advised that, when these symptoms set in, the progressives lenses are removed for a short period and replaced after symptoms have subsided. Returning to an older prescription or different type of lens design(bifocal, trifocal) only serves to increase the adaptation period to the Progressive Lenses.
- Depth perception and distance estimation can be influenced during the accommodation period.
